The present invention relates generally to an antenna system and, more particularly, to an improved antenna system for ground applications.
The operation of Remote Satellite Measurement Units (RSMUs) with Satellite Landing System (SLS) ground stations is particularly susceptible to multi-path errors. Such errors typically originate from the illumination of the antenna by rays that are reflected from the earth or surface objects and structures. Thus, it is desirable to design a ground-based system that would acquire emissions that originate above the horizon, but reject rays that arrive from below the horizon. Antenna systems that exhibit such a sharp radiation cut-off pattern are typically very large.
Previous attempts to reduce multi-path errors have employed L-band antenna designs. These efforts have met with limited success. Early trials involved patch antennas and quadri-filar helix designs. To help improve the performance of these antennas, choke rings were introduced around the bases of the basic antenna elements in an attempt to reduce the response to signals that reflect from the earth and other objects below the horizon. In addition, xe2x80x9clift kitsxe2x80x9d have been installed with patch antennas to raise the patches to various heights above the choke ring base of the antenna. While some of these trials have met with limited success, none have satisfactorily eliminated the multi-path errors.
The latest attempt to reduce multi-path errors uses a large array having a vertical array of vertically polarized dipoles and a second antenna which is a heli-bowl mounted above the vertical dipole array. The vertical dipole array provides coverage of lower elevation angles and cuts off sharply below an elevation of approximately 5-10 degrees. Furthermore, the vertical dipole array also cuts off at higher elevation angles in the range of about 35-40 degrees above the horizon. As may be appreciated, coverage of elevation angles near the zenith would be fundamentally limited with the vertical dipole array as the vertical dipole elements do not radiate or receive in the vertical direction.
Regardless of the array construction, coverage typically will not be provided for a direction in which the basic elements do not radiate or receive. Therefore, two antenna sections are configured for coverage of the low elevation angles and the high elevation angles. More specifically, the vertical dipole array is provided for the low elevation angles and the heli-bowl is provided for the high elevation angles.
A two antenna configuration, including the heli-bowl and vertical dipole array combination, typically requires the use of two separate receiver channels. The signals from the two antenna sections cannot be combined into a single analog or digital signal prior to signal detection because at some elevation angles, the summation of two Radio Frequency (RF), Intermediate Frequency (IF), or digital signals will result in a signal aiding or cancellation in the common region where the radiation patterns of the two sections overlap. This results in undesirable pattern nulls and peaks commonly referred to as grating lobes. While the situation involving peaks in an antenna pattern due to signal aiding is not generally considered to be a problem, nulls resulting from signal cancellation are undesirable due to a reduction in coverage volume.
In addition to the disadvantages associated with signal cancellation in a two antenna configuration, the required use of two receivers for this antenna type imposes a cost penalty. For example, a single SLS ground station is typically outfitted with three RSMUs. Therefore, a two antenna configuration would typically require six receivers for each SLS ground station. In addition, synchronization between multiple RSMUs at each site must be resolved, and a switching threshold algorithm is needed to select the proper receiver output based on elevation angle, signal quality, or some other appropriate parameter.
U.S. Pat. No. 5,534,882, issued Jul. 9, 1996 to Lopez, discloses an antenna system having upper hemisphere coverage close to the zenith and is hereby incorporated by reference. Referring to FIG. 1, a computer-generated plot of antenna gain versus elevation angle for the Lopez system is illustrated. As shown, the gain is uneven from the horizon to the zenith (0 to 90 degrees). There is a sharp cutoff at the horizon with the sidelobes approximately 10 dB+ down.
In view of the foregoing, an antenna array is desired that is constructed with basic elements and provides improved isotropic coverage of the upper hemisphere while rejecting signals that arrive from below a suitable threshold above the horizon (i.e., upper hemisphere coverage with a sharp cut-off near the horizon). In addition, it is desirable to stabilize the gain from the horizon to the zenith.
Various embodiments of the present system overcome the prior art problems by providing an improved antenna comprising a plurality of vertically-distributed element arrays configured to cover the upper hemisphere while providing a sharp cut-off at a relatively small angle above the horizon.
In accordance with a further aspect of the present invention, an antenna includes a plurality of element arrays distanced by at least xcex1xcex/2, wherein xcex1 is an unitless constant and xcex is the wavelength.